1. Field of the Invention
The present disclosure relates generally to input/output processing, and in particular, to reducing device contention issues associated with multiple requests to access a reserved device.
2. Description of Background
Input/output (I/O) operations are used to transfer data between memory and I/O devices of an I/O processing system. Specifically, data is written from memory to one or more I/O devices, and data is read from one or more I/O devices to memory by executing I/O operations.
To facilitate processing of I/O operations, an I/O subsystem of the I/O processing system is employed. The I/O subsystem is coupled to main memory and the I/O devices of the I/O processing system and directs the flow of information between memory and the I/O devices. One example of an I/O subsystem is a channel subsystem. The channel subsystem uses channel paths as communications media. Each channel path includes a channel coupled to a control unit, the control unit being further coupled to one or more I/O devices.
The channel subsystem may employ channel command words (CCWs) to transfer data between the I/O devices and memory. A CCW specifies the command to be executed. For commands initiating certain I/O operations, the CCW designates the memory area associated with the operation, the action to be taken whenever a transfer to or from the area is completed, and other options.
During I/O processing, a list of CCWs is fetched from memory by a channel. The channel parses each command from the list of CCWs and forwards a number of the commands, each command in its own entity, to a control unit coupled to the channel. The control unit then processes the commands. The channel tracks the state of each command and controls when the next set of commands are to be sent to the control unit for processing. The channel ensures that each command is sent to the control unit in its own entity. Further, the channel infers certain information associated with processing the response from the control unit for each command.
Performing I/O processing on a per CCW basis may involve a large amount of processing overhead for the channel subsystem, as the channels parse CCWs, track state information, and react to responses from the control units. Therefore, it may be beneficial to shift much of the processing burden associated with interpreting and managing CCW and state information from the channel subsystem to the control units. Simplifying the role of channels in communicating between the control units and an operating system in the I/O processing system may increase communication throughput as less handshaking is performed.
Additional problems can arise in managing requests from channels controlled by multiple operating systems to command a common I/O device via a control unit. The multiple operating systems can exist upon a common host system or across multiple host systems, with each host system including a channel subsystem and processing elements. When multiple operating systems attempt to access a common I/O device that has been reserved, the control unit typically receives a device busy indicator from the I/O device and reports the device busy indicator to the channels controlled by the operating systems requesting access. The access request may be a command to perform an I/O operation with or without reservation. Once the I/O device becomes non-busy, the control unit sends a device end indicator to the operating systems via their respective channels to notify them that the I/O device is available. The channel subsystems can then male the previously attempted request again, with the first-in-time channel winning the race condition relative to the other channels contending to access the I/O device. A faster responding host system can effectively block out slower responding host systems, as reservation requests are granted to the first-in-time requester. For example, an operating system that is a running on a host system that is further in distance from the I/O device may be prevented from accessing the I/O device for long periods of time, as an operating system running on a host system that is closer to the I/O device experiences a shorter communication transport delay. Thus, as contention for reserving the I/O device and subsequent access requests increases, the disparity between operating systems in accessing the I/O device also increases. Accordingly, there is a need in the art for reserved I/O device contention reduction at a control unit in communication with a plurality of operating systems via one or more channels.